1. Field of the Invention
The present invention relates to wireless communications. More specifically, the present invention relates to interference reduction for co-channel multi-carrier and narrowband wireless communication systems.
2. Discussion of the Related Art
In the near future, third generation (3G) wireless communication systems will transition to fourth generation (4G) wireless communication systems. Two promising physical (PHY) layer technologies for 4G implementation are LTE and WiMAX, which are both multicarrier systems each having a bandwidth as wide as 20 MHz. As compared to these promising technologies, the 5-MHz bandwidth 3G systems (e.g., EDGE, DECT, CDMA-2000, and W-CDMA) are considered narrowband (NB) systems. During the transition phase from 3G to 4G wireless communication systems, some multicarrier and NB systems may need to share the same spectrum. Coexistence of multicarrier and NB wireless communication systems may result in performance degradation in both systems due to co-channel interference (CCI).
FIG. 1 shows a wireless communication system in which narrowband system 50 (e.g., a W-CDMA based femtocell network), and wideband system 20 (e.g., a macrocell network) coexist within service area 60 of wideband system 20, sharing a communication channel. Under such an arrangement, interference exists between the femtocell mobile stations (fMSs) and the macrocell base station (mBS), as indicated by reference numeral 30. In addition, interference exists between the femtocell base station (fBS) and some of the mobile stations of wideband system 20, as indicated by reference numeral 40. As macro-cellular networks migrate to wideband multicarrier-based technologies, and while existing 3G femtocells migrate slowly to 4G, a “Long Term Evolution” (LTE) based macrocell may coexist with 3G femtocells within the macrocell's coverage area. For improved performance, fMSs preferably cancel interference 30. Similarly, to improve performance, interference 40 may be mitigated at MS 10. Other similar conditions in the uplink where the narrowband users interferes with the fBS, and the fMS interferes with the mBS can be easily analyzed, and mitigation of interference would also improve the performance under these conditions.
Recently, femtocells have gained considerable attention and several trial deployments have been reported by different operators. Initial deployments of femtocells are based on CDMA-based technologies (e.g., W-CDMA). The article, “Uplink Capacity and Interference Avoidance for Two-Tier Cellular Networks” (“Chandrasekhar”) by Vikram Chandrasekhar and Jeffrey G. Andrews, published in Proc. IEEE Global Telecommunications Conference (GLOBECOM), pp. 3322-3326, November 2007, discloses two options for femtocell deployment. According to Chandrasekhar, femtocell users and macrocell users in a split spectrum network may use orthogonal subchannels. However, while interference between the macrocell and the different femtocells is minimal because of the use of the orthogonal subchannels, the spectrum is not efficiently utilized. In contrast, in a shared spectrum network, femtocell users may use subchannels that are already being used by the macrocell (i.e., a co-channel operation) under certain conditions. Despite the possibility of interference, which may be insignificant if the fBS is far away from the mBS, co-channel femtocell deployment are advantageous because of a greater and more efficient spectrum utilization, and a simpler cell-search process.
Using orthogonal frequency division multiplexing (OFDM) to suppress narrowband interference (NBI) is discussed, for example, in the articles (a) “A rank-reduced LMMSE canceller for narrowband interference suppression in OFDM-based systems” (“Nilsson”), by R. Nilsson, F. Sjoberg, and J. LeBlanc, published in IEEE Trans. Commun., vol. 51, no. 12, pp. 2126-2140, December 2003; (b) “Narrowband interference in pilot symbol assisted OFDM systems” (“Coulson”), by A. Coulson, published in IEEE Trans. Commun., vol. 3, no. 6, pp. 2277-2287, Nov. 2004; (c) “Narrowband interference rejection in OFDM via carrier interferometry spreading codes” (“Wu”), by Z. Wu and C. Nassar, published in IEEE Trans. Commun., vol. 4, no. 4, pp. 1491-1505, July 2005; and (d) “A novel narrowband interference canceller for OFDM systems” (“Zhang”), by D. Zhang, P. Fan, and Z. Cao, published in Proc. IEEE Wireless Commun. and Network. Conf. (WCNC), vol. 3, March 2004, pp. 1426-1430.
Nilsson uses linear minimum mean-square error (LMMSE) estimates of interference. Nilsson's algorithm requires à priori information about the power spectral density of the NB signal. In Coulson, a normalized least mean squares (N-LMS) adaptive noise cancellation algorithm suppresses NBI in a pilot symbol-assisted OFDM system. Wu discloses NBI rejection using interferometry spreading codes. Zhang discloses an NBI canceller for an OFDM system in which the NB signal is estimated over unused OFDM subcarriers. Zhang's method is limited in practice because of the small number of unused subcarriers in a well-designed OFDM based system.
Iterative methods for mitigating CCI are disclosed, for example, in the articles (a) “Cochannel interference suppression with successive cancellation in narrow-band systems” (“Arslan”), by H. Arslan and K. Molnar, published in IEEE Commun. Lett., vol. 5, no. 2, pp. 37-39, 2001; and (b) “Iterative semi-blind single-antenna cochannel interference cancellation and tight lower bound for joint maximum-likelihood sequence estimation” (“Schoeneich”), by H. Schoeneich and P. Hoeher, published in Signal Proc., vol. 84, no. 11, pp. 1991-2004, 2004. Both Arslan's and Schoeneich's systems are narrowband only. Arslan, for example, teaches exploiting the differences in signal features (e.g., relative delay) to obtain an initial signal separation, which can considerably increase iterative interference cancellation efficiency.